Typical inkjet inks employ anionic dyes that are neutralized with cations such as Na+ and/or Li+. The demand for fast drying inks has also led to an increase in the use of organic solvents in the ink. However, the limited range of miscibility of these organic solvents in an aqueous-based ink often leads to phase separation, especially at higher temperatures, and consequently reduces the solubility of the dye. This low solubility of the dye in turn degrades the decap performance of the inkjet pen. Additionally, because many of the organic solvents often have a greater boiling point than the corresponding aqueous-based inks, evaporation of water during inkjet drop ejection also leads to precipitation of the dye. This in turn increases the dye concentration of the ink around the nozzle portion of the inkjet pen, thereby saturating the ink vehicle. Because this phenomenon takes place near the nozzle portion of the inkjet pen, it also degrades decap performance. Short term decap performance refers to the time period in between successive firings a nozzle can tolerate without a defect. Long term decap performance refers to the level of nozzle recovery after the nozzles have been idle for an extended period of time. Use of counterions, such as NH4+, in ink formulations can improve the decap performance of inks for inkjet printing. Still, it is desirable to improve the solubility of dyes in organic-based inks without suffering from the disadvantages of decreased decap performance (U.S. Pat. Nos. 6,221,143, issued Apr. 24, 2001, and 5,830,265, issued Nov. 3, 1998, and PCT Publication WO 01/25340, published Apr. 12, 2001).
Solubility is proportional to the strength of the non-covalent molecular interactions between the solute and the solvent molecules. Increased ink solubility in organic-based vehicles results in part from an increase in the hydrophobic interactions between the dye and the ink vehicle. The increased hydrophobicity of the dye molecules that increases solubility in organic-based vehicles also enhances interactions between the dye and hydrophobic print media, such as offset coated papers. These interactions increase the durability of the printed ink on such media; however, customers do not purchase an ink with the intent to use it solely on hydrophobic papers. Thus, it is desirable to formulate a dye that is soluble in organic-based vehicles but leads to greater durability properties (such as waterfastness, smearfastness and smudgefastness) when printed on either hydrophilic media (plain paper) or hydrophobic media (coated offset media).